Arguments in support of any particular superconducting coating must be framed in terms of its fundamental thermodynamic properties. The superconducting transition temperature, T c , determines the surface resistance, and thus the Q of the cavity. This must remain sufficiently high that the system can be driven at the required field gradients and frequencies without leading to excessive power loss. In this regard the 39 K T c of MgB 2 is advantageous. With an anticipated maximum accelerating

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... d, E MAX acc , of 77 MV m −1 and a BCS surface resistance, R BCS s (4 K, 500 MHz), of 2.5 n as discussed later, MgB 2 represents an interesting possibility as a coating for SRF cavities. In addition, the higher H c2 of MgB 2 than Nb results in a slightly lower estimated trapped flux sensitivity. Recent measurements of an MgB 2 film at the Los Alamos National Laboratory (LANL) have shown an RF surface resistance lower than that of Nb at 4 K, which is proof-of-principle evidence of the attractiveness of MgB 2 . Our calculations are based conservatively on 4 K operation at 500 MHz. However, with a T c of 39 K, MgB 2 -coated cavities should be less susceptible to thermal breakdown than low-T c ones. Superconducting materials for use at GHz frequencies at voltage gradients >40 MV m −1 , a recently cited target, will require both low R s (high T c ) and high H sh values. With a T c of 39 K, MgB 2 clearly has the potential to reduce R BCS s if the films are well prepared and free from defects and particles. Additionally, while the H c1 for MgB 2 is relatively low, the superheating critical field, H sh , is higher than that of Nb. Currently, there is some debate about the exact roles of H c1 and H sh in the determination of E acc limits. However, the higher values of H sh for MgB 2 do suggest the possibility of enhanced E acc values. The exact roles of H c1 and H sh should be further investigated. Techniques exist that may enable cavity-like structures to be internally coated with a MgB 2 film.